From Glossary of Meteorology
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(Usually means linear momentum as opposed to angular momentum.) In Newtonian mechanics the (linear) momentum p of a body with mass m and velocity v is the product of these two quantities:

In the absence of forces, momentum is conserved. But momentum is a more fundamental quantity than simply the product of mass and velocity. For example, photons have momenta, which can be transferred to objects (as evidenced by radiation pressure), and yet the photon has zero rest mass. Thus, momentum is best looked upon as a single entity, complete in itself, governed by the dynamical law
where F is the force acting on the body with momentum p.